1. Technical Field
The present invention generally relates to wireless receiver systems and in particular to error compensation in wireless receiver systems.
2. Description of the Related Art
Uplink (UL) spatial division multiple access (SDMA) or collaborative multiple inputs multiple outputs (MIMO) system is an important feature in Worldwide Inter-operability for Microwave Access (WiMAX) which allows wireless communication devices of two mobile users to share the same radio resource at the same time. As a result, system capacity is theoretically doubled. In reality, channel information is unknown and timing errors and frequency errors are present in the received signal from each mobile user. A practical UL SDMA receiver must address the critical issues of channel estimation, timing error correction and frequency error correction.
A traditional minimum mean square error (MMSE) receiver/equalizer is the simplest receiver to implement and provides acceptable performance with perfect time/frequency synchronization. However, the MMSE receiver frequently experiences large performance loss. For example, performance of the MMSE receiver in the presence of 16-QAM (Quadrature Amplitude Modulation) degrades rapidly with modest timing or frequency offset. Conventional MMSE equalizers are unable to achieve the performance requirements in the presence of residual synchronization errors after ranging (i.e., estimating timing errors via training signals), especially for a WiMAX defined signal structure.